1. Field of the Invention
The present invention relates generally to the field of high-purity plumbing systems, and more particularly to the field of plumbing high-purity systems in the fields of semiconductor processing, biopharmaceuticals, and the like.
2. Description of the Background Art
High purity plumbing systems face many constraints that are not major concerns in common plumbing systems (e.g., commercial and residential water supply systems). Of particular relevance to the present invention is the requirement that the inner walls of the pipes in high-purity systems be extremely smooth. In bio-pharmaceutical systems, rough surfaces inside pipes can harbor dangerous bacteria. In semiconductor processing systems, rough internal pipe surfaces cause an undesirable increase in particulates and outgassing. Therefore, to be acceptable for use in high-purity systems, it is generally accepted that the inner surfaces of pipes must have a roughness average (Ra) value of 25 xcexcm or less.
Bending a pipe causes a surface roughness effect, known in the art as xe2x80x9corange peelxe2x80x9d, on the pipe""s inner surface. For this reason, it is well accepted in the art that bent pipes are unsuitable for use in high purity systems, absent some post bend treatment. Instead, high purity systems are constructed by welding fittings to the ends of straight runs of pipe.
The fittings themselves are manufactured by bending pipe, but substantial post bend processing is required. The fittings are manufactured from pipe having a greater wall thickness than the pipe they will join, because the post bend polishing required to smooth the inner surface of the fitting removes a portion of the wall thickness. Further, because bending induces stresses in a pipe, a post-bend heat treatment, or anneal, is required to alleviate those stresses.
Flanges are welded to the fittings to facilitate coupling the fittings to other pipes. Welds are intrusive and cause metallurgical changes in the fitting, which result in diminished corrosion resistance. The welds also cause physical changes in the fittings, which disrupt the smoothness of the inner surface. Therefore, post-weld processing is also required to minimize these undesirable effects of welding.
Standard off-the-shelf fittings in the field are generally limited to angles of 45xc2x0 and 90xc2x0. Further, due to space concerns, it is generally understood that the fittings should have as tight a bend as possible. Therefore, standard fittings are generally bent such that the radius of the bend is 1xc2xd times the diameter of the tube (1xc2xdD). This value 1xc2xdD is standard in the industry because it is generally accepted that this is the tightest bend that can be produced without significant complications (e.g., pipe collapse, etc.). What is needed is a system and method for bending pipe that satisfies the rigid requirements of high purity systems, without the need for post-bend processing. What is also needed is a system and method for minimizing the number of welded fittings required in a high-purity fluid handling system. 
The present invention overcomes the problems associated with the prior art by providing a method of bending pipes such that the bent pipes are acceptable for use in high-purity fluid handling systems without the need for post-bend heat treatment or polishing. The invention facilitates the use of bent pipes, instead of welding fittings, in high-purity systems, resulting in a substantial savings in labor and materials.
A method of constructing a high-purity fluid handling system according to the present invention includes the steps of providing a metallic tube having a diameter (D) and a grain-size number of at least 5, forming a bend in the metallic tube, and installing said tube in said highpurity fluid handling system (e.g., semiconductor processing system, bio-pharmaceutical system, etc.). The radius of the bend in the tube is greater than one and one-half times the diameter (1.5D) of said tube. In a particular method, the radius of the bend is between 1.5D and 3D, and in an even more particular embodiment, the bend is a 2D bend.
Bends can be formed, according to the present invention, having any angle, thus providing an advantage over the prior art where typically only fittings having common bend angles (e.g., 30xc2x0, 45xc2x0, 90xc2x0, etc.) are commercially available. Further, multiple bends can be formed in a single pipe. Optionally, a non-metallic mandrel is used when forming the bends, to avoid marring the inner surface of the pipe.
In the particular disclosed methods, the tubes bent are constructed from stainless steel. Generally, the bent pipes are suitable for installation in a high-purity system without any post-bend heat treatment or polishing. However, if a system has particularly rigorous inner surface smoothness requirements, then the bent pipes can be subjected to an electropolishing process prior to being installed in the system.
A system incorporating pipes bent according to the present invention is also described. The system includes a chamber and a high-purity line in fluid communication with the chamber. The high-purity line is formed from a metal having a grain-size number of at least 5, has a diameter (D), and includes at least one bend having a radius greater than 1.5D. In one particular embodiment, the high-purity lines are main (e.g., diameter xe2x89xa71 inch) fluid supply lines, individual (diameter  less than 1 inch) fluid supply lines, and vacuum forelines of a semiconductor processing system. Optionally, the high-purity lines are any line coming into contact with product inside a bio-pharmaceutical processing system.